1. Field of the Invention
The present invention relates to apparatus for determining the liquid level in a tank and, more particularly, to externally mounted means for determining and providing a remote indication of the liquid level in a tank, such as a waste water tank for a recreational vehicle.
2. Description of the Prior Art
A wide variety of different types of liquid containing tanks are provided with measurement means for determining the liquid level therein. Usually, such measurement means are positioned within the tank and directly contact the surface of the liquid in order to determine the level thereof. On the other hand, in certain cases, such as in the measurement of the liquid level in waste water holding tanks used in recreational vehicles and the like, it is desirable that the measurement means not require an object to contact the liquid inside the tank or require holes to be drilled in the tank. Any obstruction or metallic object inside of a waste water tank will be subject to strong corrosive action and will become entangled with solid or semi-solid materials, causing a malfunction of the measuring device and/or preventing the tank from being completely emptied. This also leads to sanitary problems and unpleasant odors in the case of waste water holding tanks.
In order to determine the liquid level in a tank without contacting the liquid therein, it has been proposed to mount metal electrodes, probes, or plates on the tank, which plates are insulated from the liquid therein in a manner such that the capacitance between the plates varies as a function of the level of the liquid in the tank. This change in capacitance can then be utilized to provide an indication of liquid level. However, several problems have been encountered with this type of system which has rendered it impractical for use heretofore.
For example, it is desirable to locate the electronic circuit components at a central location, preferably where the liquid level indicating means is located. This permits the electronic circuit components to be used for measuring more than one tank such as the holding tank and the water tank for a recreational vehicle. This is also desirable so that the electronic components are in a more benign environment and to eliminate the cost of special containers to protect the electronics when mounted externally. However, when this is done, a cable is required for connecting the plates to the electronics. The problem is that the capacitance of the cable tends to be significantly more than the capacitance between the plates, making accurate measurements extremely difficult since the major portion of the current is dissipated by the cable capacitance. As a result, it has been necessary heretofore to mount the electronics adjacent the tank with the attendant disadvantages discussed previously.
In a practical system, the measuring circuitry should be relatively independent of variations in conductivity of the liquid in the tanks since some liquids, such as waste water, will be highly conductive while drinking water may be quite pure and have a very low conductivity. Most circuits designed heretofore have not had this relative independence.
The measurement circuitry should also be relatively independent of operating frequency so that frequency stability of the oscillator used to drive the plates is not a factor in the accuracy of the measurement. Again, in circuits designed heretofore, the circuit has not had this relative independence.